Couch potato

: : : : Armchair XXX : Someone who sits at home in their comfy chair, perhaps knows very little about a subject, yet constantly offers their opinion as if they are an expert. The most common in the US is the armchair quarterback who watches football on TV and tries to tell his team what they are doing wrong and what they should be doing right. The armchair quarterback is frequently married to the back-seat driver. (^_^)

: : : In my student days, people who talked about the need for capitalism to be overthrown but who wouldn't attend street marches (which were at that time illegal in the part of Australia where I live) were called "armchair socialists". pamela

: : And in the interests of candor, I must admit that I, too, am an archair sociologist. I observe my fellow creatures and speculate about the reasons for this or that social activity. I sometimes even require my friends and companions to listen to my theories. This has sometimes been useful to them when they needed a soporific. SS

: I'm waiting for "armchair psychiatrist" to turn up. ~rb

shouldn't it be 'couch psychiatrist'?

you could have 'armchair dentists' but would an 'armchair furniture restorer' be an amateur or a professional?